Lofting 101

My last blog was on November 15 when we were preparing the shop floor for lofting. Since then we have worked continuously on lofting "The Sixteen" with the exception of the Thanksgiving break. I will attempt to describe what I have been doing without writing a short novel. To be honest I find lofting to be fun but lofting is also tedious, frustrating and at times just plain confusing (and why I have probably not blogged for two weeks). The good news is that other than some beginning projects that may relate to boatbuilding (rabbet, carlin, etc.) this is the start of real boatbuilding. Drafting and recreating the lines of the boat in full size on the lofting floor is only a fraction of the boatbuilding process. The process really begins to take form in creating developments and molds.

The lectures on lofting which have spanned several weeks now were given by the school's senior instructor and master boatbuilder. Jeff has been a boatbuilder for 30+ years and apprenticed under the school's founder, Bob Prothero. His experience and knowledge was further demonstrated in the fact that all of the instructors attended his lectures along with the students. One of the first things Jeff said when talking about boatbuilding is to “strive for perfection and settle for a little less”. I think this is great advice for someone new to boat building and maybe for life in general.

Some of the insight Jeff passed along in his discussion on lofting and boat building:

1. Grid lines are actually planes. The line defines the boundary of the plane.
2. Atkins plans have errors in the table of offsets (you have been forewarned).
3. Waterlines, butt lines and diagonals are just tools to fair the body plan lines. Diagonals help square off the body plan. The body plan is the most important of all the lines because it gives shape to the hull. The body plans contain 85-90% of the useful information.
4. The Sixteen is similar to a Herreshoff 12 ½.
5. Fairing lines is similar to music but using your eyes instead of your ears.

Skeg built vs. Built down (what is done out here)

Built Down: constructed with the frames or hull bottom joining the keel in a plane nearly parallel with the sides of the keel, forming an S-curve from rail to keel.

Skeg Built: constructed with the frames or hull bottom joining the keel in a plane nearly perpendicular to the sides of the keel

Skeg built boats are said to be more stable with large loads, faster and generally lighter while built down boats are said to be stronger and more seakindly. Both sides argue that one is superior than the other.

Difference between East Coast and West Coast boatbuilding. In the East they use a steeper dead rise due to the higher modulation of the wave action.

Boatbuilding Methods

A. Lofting lines – Developments

B. Spiling – using cheap material to fit a piece
C. Scribing – transfer shape from one piece to another, directly on material, one direction only
D. Fitting

Developments

A. Bearding line – inside surface of planking where it enters the backbone.
B. Rabbet – outside of the planking.
 1. Stem section:
  - Partial development of new body plan line squared off from the rabbet

  - New plane with different orientation perpendicular to the rabbet

C. Mold reduction - the depiction of planking (constant thickness), framing (constant thickness) and longitudinals. Expansion from outside planking to the edge of mold.
 1. Deck Camber:

  - Use maximum beam width
  - Rise = 1/2” per 1-0-0 of beam (Herreshoff = 3/8”/Culler = 3”) rounded up 2. Longitudals:
  - Taper to half of midships thickness to be able to bend.
  - Spar Taper – divide length into four equal parts 3. Bevels:
  - Make a bevel stick = 2x station length (48”) from the master bevel board, square off on station and read bevel on next station

  - For Sta 4 move Sta 5 forward

  - For Sta 1 add Sta 0.5 and Sta 1.5 (changed station distance – need new bevel board)

  - For Sta 7 measure 2” aft of 7 to create Sta 7.2, develop new body plan line
  - Why the bevel stick works? Difference in breadth between diagonals is equal to the bevel angle.
  - Clear bevel board (Mylar) width = at very least the total maximum mold reduction, parallel edges and approximately 18” long and up to 30 degrees

  - Measure total mold reduction for Sta_ on clear bevel board and use to mark mold.
  - Deck Centerline - determine the bevel on mold (at notch) from the shelf

  - Loft shelf and clamp vs. Spilling

  - Top of apron = 1 ¼” above back rabbet

D. Transom (curved, raked)

The elusive but most important book related to lofting:

Ship and Aircraft Fairing and Development, for Draftsmen and Loftsmen and Sheet Metal Workers by Samuel Supplee Rabl